MarkB
Legend
Something that struck me as funny both times I saw this movie:
Stark (paraphrased): "It's made of vibranium. It doesn't vibrate." At that moment and a couple more times in that scene, you hear the shield vibrating.
Then several times throughout the rest of the movie, you hear the shield vibrating.
If the sound effects guys couldn't just leave the shield sounds out, then at least the editors could have taken out the "It doesn't vibrate," line.
Bullgrit
It's not "it doesn't vibrate", it's "it absorbs vibration". So when Peggy shoots it point-blank, the bullets' momentum is absorbed and they fall to the floor instead of bouncing back and killing her or Stark. Likewise, the energy of Hydra's cube-powered weapons gets absorbed instead of disintegrating the shield and Cap.
There's nothing to say that the shield itself won't vibrate - in fact, given all the kinetic energy it's absorbing, it's a testament to Cap's strength that it doesn't vibrate more.